feat(settings): configurable monitoring thresholds

Move the hardcoded warn/crit cutoffs into Settings -> Thresholds (DB-backed,
live, no restart). New thresholds.* keys + to_thresholds_cfg() + a
threshold_style(value, kind) jinja global that reads them; latency reuses the
existing ping good/warn keys, uptime is direction-aware (floors).

Replace the _macros metric_style/uptime_style macros (now removed) with the
global across Hosts-Overview, host_agent fleet + panel, Uptime/SLA widget, and
the ping page uptime column — all now honor the configured cutoffs. Uptime keeps
its green 'good' look when not degraded.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

steward/templates/hosts/uptime_widget.html

@@ -26,14 +26,11 @@
     <span style="text-align:center;font-family:ui-monospace,monospace;font-size:0.78rem;">
       {% if pct is none %}
         <span style="color:var(--text-dim);">—</span>
-      {% elif pct >= 99.9 %}
-        <span style="color:var(--green);">{{ "%.1f"|format(pct) }}%</span>
-      {% elif pct >= 99 %}
-        <span style="color:var(--yellow);">{{ "%.1f"|format(pct) }}%</span>
-      {% elif pct >= 95 %}
-        <span style="color:var(--orange);">{{ "%.1f"|format(pct) }}%</span>
       {% else %}
-        <span style="color:var(--red);">{{ "%.1f"|format(pct) }}%</span>
+        {# Configurable uptime thresholds (Settings → Thresholds); a normal value
+           (no degraded style) stays green to read as healthy at a glance. #}
+        {% set us = threshold_style(pct, 'uptime') %}
+        <span style="{{ us if us else 'color:var(--green);' }}">{{ "%.1f"|format(pct) }}%</span>
       {% endif %}
     </span>
     {% endfor %}